Chronic wounds—wounds that do not heal in months or even years—are one of the most persisting medical challenges because of their vast influence on public health [1, 2]. Standard treatment approaches including debridement of the necrotic tissue, maintenance of a moist wound bed, and control of the infection often do not produce the desired result. Wounds in patients with diabetes mellitus represent an even bigger problem since the healing process in these patients is known to be impaired .
Therefore, additional treatment options such as negative pressure dressings, hyperbaric oxygen therapy, topical application of carbon dioxide, and light therapy  are often employed. Light therapy has increasingly been investigated ever since Mester incidentally discovered that low-level laser therapy (LLLT) accelerated hair regrowth in laser-irradiated rats .
The effect of LLLT was first investigated in vitro to verify the influence of LLLT on cell proliferation [5, 6, 7]. Fibroblasts in cell cultures that were stimulated with LLLT proliferated significantly faster compared to sham-irradiated control cells [8, 9]. Other cell lines (gingival and mucosal fibroblasts, keratinocytes, osteoblasts, etc.) also showed faster proliferation if treated with LLLT [10, 11]. The next step in investigating LLLT was in vivo experiments on animals (mostly rats), which showed faster wound healing following LLLT [8, 10, 12]. The encouraging results of preclinical studies prompted the introduction of LLLT to different fields of medicine (wound healing, rheumatology, oral and sports medicine, etc.) [13, 14, 15, 16].
A recent survey critically reviewed eight clinical studies investigating the influence of LLLT on the healing of diabetic foot ulcers; all of the reviewed studies confirmed a beneficial effect of LLLT on the healing of diabetic ulcers .
Conversely, studies presenting data on LLLT and wound healing in general did not give such convincing conclusions. In his review in 2008, Sobanko concluded that LLLT in humans does not improve wound healing and advised better controlled studies in humans to determine the appropriate laser parameters and treatment protocol . Kilik, on the other hand, confirmed that LLLT improved wound healing in normal and diabetic rats .
Wound healing in diabetic patients is probably impaired due to hyperglycemia, inhibition of inflammatory response, poor angiogenesis, fibroplasia and defects in collagen deposition, and differentiation of the extracellular matrix [1, 5].
The exact mechanism of low-power laser effect on tissue healing is not yet completely understood.
Studies have shown that LLLT accelerates the respiratory chain and increases reactive oxygen species (ROS), NO, and intracellular Ca2+ in stressed and hypoxic cells, but not in healthy cells [5, 19, 20]. Is it possible that the beneficial effect of LLLT on the wound-healing process in diabetic patients is more pronounced compared to non-diabetic patients because their cells are additionally hypoxic and stressed due to the diabetes itself?
The process of wound healing goes through the phases of inflammation, proliferation, and maturation . A sufficient blood supply is mandatory for wound healing, but it is impaired in diabetic and non-diabetic patients with chronic wounds. This should be kept in mind when interpreting the results of published clinical studies.
The term LLLT was used for laser light only until the National Aeronautics and Space Administration (NASA) developed a new generation of light-emitting diodes (LEDs) to accelerate plant growth during space flights [21, 22]. Accelerated wound healing in astronauts treated with LED encouraged its use for medical purposes, and clinical experiences showed comparable results to LLLT [2, 21, 22, 23]. The abbreviation LLLT was later used for “low-level light therapy,” including low-level laser therapy or low-level light therapy using LED.
New generations of LED proved to be effective in wound healing if the right wavelengths, power density, and doses were used [24, 25]. In his review, Chaves compared the efficacy of low-level light therapy with laser and LED and concluded that both yielded similar biological effects, with no significant differences . Light from lasers is coherent while light from LED is not; however, according to Karu, coherence is lost during the interaction of light with biological tissue and thus is not a prerequisite for the process of photostimulation or photoinhibition .
Results from previous studies evaluating the effect of LLLT on chronic wound healing in general are conflicting. In diabetic patients, LLLT was predominantly shown to be effective, whereas in non-diabetic patients, its benefits were not as pronounced. The aim of our study was to compare the influence of LLLT (using LED) as an additional therapy for chronic wound healing in diabetic and non-diabetic patients. Since a sufficient blood supply to the wound area is mandatory for healing, the microcirculation of the healthy skin on the wound margin was the main outcome measure of our study. Additionally, the wound bed score according to Falanga was evaluated .